1. Field of the Invention
The present invention relates to an organic light emitting device. More specifically, the present invention relates to a white organic light emitting device in which efficiency is improved by specifying an energy level of a hole transport layer or a blocking layer adjacent to a light emitting layer.
2. Discussion of the Related Art
The recent trend toward information-dependent age has brought about rapid development in display fields that visually display electrical information signals. In this regard, a variety of flat display devices having superior properties such as slimness, low weight and low power consumption are developed and are actively used as substitutes for conventional cathode ray tubes (CRTs).
Specific examples of flat display devices include liquid crystal display devices (LCDs), plasma display panel devices (PDPs), field emission display devices (FEDs), organic light emitting devices (OLEDs) and the like.
Among these, organic light emitting devices that do not require an additional light source, have a compact design and give clear color are considered to be competitive in the flat display device market.
Organic light emitting display devices require the formation of organic light emitting layers. The formation of organic light emitting layers is generally carried out by a deposition method using a shadow mask.
However, shadow masks which cover a large area may be bent due to load. For this reason, it is impossible to use the large-area shadow mask several times, and defects associated with pattern formation of organic light emitting layers occur. Accordingly, there is a demand for an alternative method.
A tandem organic light emitting device (hereinafter, referred to as “tandem organic light emitting device”) suggested as an alternative to the shadow mask organic light emitting device will be described below.
The tandem organic light emitting device is characterized in that respective light emitting diode layers interposed between an anode and a cathode are deposited without using a mask, and organic films including the organic light emitting layers are sequentially deposited under vacuum using different materials.
Meanwhile, realization of white light using a tandem organic light emitting device may be performed by mixing light emitted from two or more light emitting layers. In this case, the tandem organic light emitting device includes a plurality of light emitting layers that emit light with different colors that are interposed between an anode and a cathode, and a charge generation layer (CGL) provided between respective light emitting layers. Stacks are separated, based on each light emitting layer.
In such a tandem organic light emitting device, one material does not emit light and a plurality of light emitting layers containing light emitting materials having different photoluminescence peaks (PL peaks) at respective wavelengths emit light at different positions of the device and the light is combined and emitted.
The tandem organic light emitting device includes a charge generation layer between the different stacks. This charge generation layer transports electrons or holes to an adjacent stack. HAT-CN or MoO3 can be used as a material for the charge generation layer.
However, conventional tandem organic light emitting devices have the following problems.
The tandem device includes a charge generation layer between different stacks. These charge generation layers transport electrons or holes to an adjacent stack. In this regard, when a charge generation layer is formed using the material commonly used for the tandem device, transport of electrons or holes between the adjacent stack and the charge generation layer may be impossible due to bad interface properties of the stacks adjacent to the charge generation layer. In this case, emission of light may normally occur in the adjacent phosphorescent or fluorescent stack. Accordingly, problems such as deterioration of driving voltage and decrease in efficiency may occur. As a result, the lifespan of the tandem organic light emitting device is deteriorated.
Given the foregoing, a great deal of research to find factors that block injection of holes or electrons into the charge generation layer is underway. Recently, research to solve these problems by changing materials for the charge generation layer has been started.